Population dynamics of Daphnia galeatat in the biomanipulated Bautzen Reservoir: life history strategies against food deficiency and predation

The population dynamics and demography of Daphnia galeata was analysed in a five year study in the biomanipulated Bautzen Reservoir. Samples were taken two times a week during the period May-July in the pelagic zone of this highly eutrophic water. Major bottom-up and top-down factors were determined during the study period and analysed with regard to their influence on Daphnia dynamics and life history. Field data on fecundity and population structure of D. galeata were combined with results from life table and growth experiments performed under approximately in situ conditions to gain insight into the mechanisms leading to a midsummer decline of this cladoceran species which dominates the zooplankton community in Bautzen Reservoir. Two main patterns of Daphnia dynamics emerged: In years without a midsummer decline the population increased slowly in spring, starting from low densities. High water transparency was observed already during the build-up of the population of D. galeata. Despite considerable fluctuations, Daphnia abundance remained on a high level throughout summer. In years with a midsummer decline the population started from relatively high densities in early May and more than doubled during one week. Peak densities were reached before the clear-water stage emerged. At the end of this period the population declined to low values which lasted for the rest of the summer. Fecundity of the Population of D. galeata declined, whereas the mean egg volume increased at the beginning of the clear-water stage as a result of declining food levels. The size at maturity (SAM) remained high during this period. Additionally, juvenile growth was reduced and the age at maturity was retarded. Survival probability was low for those daphnids born shortly before or during the clear-water stage compared to those born later. It can be concluded from these results that recruitment to adult stages is strongly reduced during the clear-water stage. The end of this period is marked by an alternation in generations. Only at that time can SAM be reduced because the new generation of adults matures at a smaller size, carrying small eggs. A high impact of non-predatory adult mortality can be expected when the population is dominated by a strong peak-cohort during the clear-water stage according to recruitment patterns during the build-up of the population. The most drastic decline both of Daphnia abundance and SAM was observed in those years when the biomass of juvenile fish exceeded 20 kg ha-1 at the end of the clear-water stage. Due to gape-size limitation juvenile fish mainly feed on juvenile daphnids during this period and thus, they reinforce bottom-up effects on the Daphnia population. When fish change their size selection towards adult daphnids at the time when the new generation takes over, this seems to represent the worst case for the Daphnia population. Consequently, the timing between bottom-up effects and the feeding pressure of juvenile fish determines the extent of the decline. - (This manuscript is also available - in the form of a book - from Shaker Verlag GmbH, Postfach 101818, 52018 Aachen, Germany world-wide web address: http://www.shaker.de, electronic-mail address: info@shaker.de. It has been posted on the web sites of Dresden University of Technology with the permission of the publisher